Time to cover Golden Triangle

I know the Golden Triangle tour consists of visiting Delhi, Agra and Jaipur, what I wanted to know what how long does it take to cover this tour by car and by train.
Out of a car and train which would be the best travel option and how many days would you recommend staying at each destination?

Delhi has a lot to offer so spending at least 2-3 days there would be good, Agra's main feature is the Taj Mahal and along with this others also go and see the fort and then make their way to Fatehpur Sikri, so a total of 2 days is enough, if you want to opt out for Fatehpur Sikri then one day at Agra is enough. Jaipur is also another city where you can see a lot so again 2-3 days would be good there.
These are the minimum number of days I have mentioned, and you can extend according to your holiday duration and needs.

Either modes of transport are fine, it depends on what you prefer a train or car. Going by train would cost you a lot less than going by car that is the only difference really.
I would suggest that in Delhi you spend at least 3 days, 1 day at Agra and 3 days at Jaipur.

Golden triangle is best seen by car, as you can stop at any place you find interesting on the way which would give you extra things to see and explore. Covering Agra in one day is the best thing to do as it would give you more time at Delhi or Jaipur, don't keep a night stay at Agra. To really enjoy the holiday I would say 10 days would be good.